The present invention relates to an apparatus for purifying the exhaust gas from an engine by means of a catalyst.
While various apparatus are conventionally used to purify the exhaust gas from an engine by means of a catalyst, there is available a model in which heated air is introduced at a point upstream of the catalyst to insure that the catalyst temperature is elevated as fast as possible after engine start-up, thereby enhancing the efficiency of purification. If heated air is introduced into the exhaust pipe right after the engine is started and when the catalyst temperature and, hence, the purification efficiency are still low, the introduced hot air will heat the catalyst and, at the same time, the reaction involving the oxidation of HC and CO in the exhaust gas is accelerated by the catalyst, thereby causing a rapid increase in the temperature of the catalyst.
A conventional exhaust gas purifier of the type under consideration is described below with reference to FIGS. 2 and 3. FIG. 2 shows schematically the layout of the conventional apparatus for purifying the exhaust gas from an engine, and FIG. 3 is a graph showing the flow quantity of air that is introduced into the exhaust pipe.
In FIG. 2, reference numeral 1A denotes the engine, 2A is the transmission of engine 1A, and 3A is a suction pipe. The portion upstream of the suction path formed by suction pipe 3A communicates with the atmosphere via air cleaner 4A.
Shown by 5A is a throttle valve that controls the quantity of air to be sucked into engine 1A. Engine 1A is so adapted that it is supplied with a fuel by means of an injector (not shown). Shown by 6A is the exhaust pipe and catalyst 7A for purifying the exhaust gas by a chemical reaction is provided downstream of the exhaust pipe 6A.
Shown by 8A is an air pump for introducing air into the exhaust pipe 6A. This air pump 8A is of a mechanical type that is driven by engine 1A and the air discharge port of the pump communicates with the exhaust pipe 6A at the portion upstream of the catalyst 7A via an air introducing pipe 9A, a control valve 10A, a check valve 11A, a heater 12A, etc. The air suction port of pump 8A either opens to the atmosphere or communicates with the suction path.
Control valve 10A is so adapted that it will open and close the air channel communicating with the air discharge port of air pump 8A and the opening or closing action of the valve 10A is controlled by a control unit 13A to be described later. Check valve 11A is so adapted that it will permit the selective passage of air from air pump BA toward exhaust pipe 6A, thereby preventing the exhaust gas from leaking out of the exhaust pipe 6A to enter the air pump 8A. The heating device 12A is fitted with a heater (not shown) that generates heat when an electric current is applied and it is so adapted as to heat the air flowing through the above-mentioned air channel. The application of an electric current through the heating device 12A is controlled by the control unit 13A as is the control valve 10A.
The control unit 13A is so adapted that when the start switch (not shown) for engine 1A is turned on, it will open the control valve 10A while causing an electric current to be applied to the heating device 12A.
Being thus constructed, the exhaust gas purifier of the present invention is operated in the following manner. When engine 1A is started, air pump 8A starts to work and, at the same time, control valve 10A opens, whereupon air is ejected from air pump 8A and flows through air introducing pipe 9A, control valve 10A, check valve 11A and heating device 12A to be introduced into the exhaust pipe 6A at the point upstream of the catalyst 7A. The change in the quantity of air thus introduced is shown in FIG. 3. Symbol A in FIG. 3 denotes the time of engine start-up. As shown, the quantity of air introduced into the exhaust pipe 6A is generally constant after engine start-up if the engine is running at constant rpm.
If engine 1A is started, control unit 13A will cause an electric current to flow through the heating device 12A, so the air discharged from the air pump 8A is heated with the device 12A before it is introduced into the exhaust pipe 6A.
The heated air thus introduced into the exhaust pipe 6A is mixed with the exhaust gas in the exhaust pipe 6A and the resulting mixture flows into the catalyst 7A. When both the exhaust gas and the heated air flow into the catalyst 7A, the latter is heated by the hot air while, at the same time, HC and CO in the exhaust gas are converted to H.sub.2 O and CO.sub.2 by O.sub.2 in the heated air. In other words, the heat of the hot air and the heat of reaction are effectively used to raise the temperature of catalyst 7A as soon as the engine is started.
The problem with the exhaust gas purifier having the construction described above is that the engine 1A must be equipped with a dynamo and a battery of large capacity. This is because the heating device 12A consumes large power and an electric current is kept applied to it throughout the period from engine start-up to its stop.
The problem with the engine 1B equipped with the conventional exhaust gas purifier is that its rotational speed is prone to drop while heated air is introduced into the exhaust pipe 6B. This is because the heating device 12B consumes large power and when it performs heating, the dynamo generates more power to increase the load on the engine 1B. Stated more specifically, if engine 1B is brought to idling, its rotational speed is not usually constant and it is prone to run inconsistently. Furthermore, if the rotational speed of the idling engine decreases, the electric power to be charged into the battery tends to be insufficient.
A conventional exhaust gas purifier of the type under consideration is described below with reference to FIGS. 7 and 8. FIG. 7 shows schematically the layout of the conventional apparatus for purifying the exhaust gas from an engine, and FIG. 8 is a graph showing the flow quantity of air that is introduced into the exhaust pipe.
In FIG. 7, reference numeral 1C denotes the engine, 2C is the transmission of engine 1C, and 3C is a suction pipe. The portion upstream of the suction path formed by suction pipe 3C communicates with the atmosphere via air cleaner 4C.
Shown by 5C is a throttle valve that controls the quantity of air to be sucked into engine 1C. Engine 1C is so adapted that it is supplied with a fuel by means of an injector (not shown). Shown by 6C is the exhaust pipe and catalyst 7C for purifying the exhaust gas by a chemical reaction is provided downstream of the exhaust pipe 6C.
Shown by 8C is an air pump for introducing air into the exhaust pipe 6C. The air pump 8C is of a type that is driven electrically and the air discharge port of the pump communicates with the exhaust pipe 6C at the portion upstream of the catalyst 7C via an air introducing pipe 9C, a control valve 10C, a check valve 11C, a heating device 12C, etc. The operation of the air cleaner pump 8C is controlled by controller 13C to be described hereinafter. The air suction port of pump 8C either opens to the atmosphere or communicates with the suction path.
Control valve 10C is so adapted that it will open and close the air channel communicating with the air discharge port of air pump 8C and the opening or closing action of the valve 10C is controlled by the controller unit 13C. Check valve 11C is so adapted that it will permit the selective passage of air from air pump 8C toward exhaust pipe 6C, thereby preventing the exhaust gas from leaking out of the exhaust pipe 6C to enter the air pump 8C.
The heating device 12C is fitted with a heater (not shown) that generates heat when an electric current is applied and it is so adapted as to heat the air flowing through the above-mentioned air channel. The application of an electric current through the heating device 12C is controlled by the control unit 13C as are the air pump 8C and control valve 10C.
The controller 13C is so adapted than when the start switch (not shown) for engine 1C is turned on, it will open the control valve 10C while causing an electric current to be applied to both air pump 8C and heating device 12C. The controll unit 13C is also adapted in such a way that when the running of the engine satisfies prescribed conditions (which are hereunder referred to as "present conditions"), it will close the control valve 10C while causing both air pump 8C and heating device 12C to stop operating. Another feature of the controll unit 13C is that when the engine 1C stops running, it will close the control valve 10C while causing both air pump 8C and heating device 12C to stop operating.
Being thus constructed, the exhaust gas purifier of the present invention is operated in the following manner. When engine 1C is started, control valve 10C is opened and, at the same time, both air pump 8C and heating device 12C start to operate, whereupon air is ejected from air pump 8C and flows through air introducing pipe 9C, control valve 10C, check valve 11C and heating device 12C to be introduced into the exhaust pipe 6C at the point upstream of the catalyst 7C.
The air ejected from air pump 8C is heated in the heating device 12C before it is introduced into the exhaust pipe 6C. The change in the quantity of air thus introduced is shown in FIG. 8. Symbol A in FIG. 8 denotes the time of engine start-up. As shown, the quantity of air introduced into the exhaust pipe 6C is generally constant after engine start-up.
The heated air thus introduced into the exhaust pipe 6C is mixed with the exhaust gas in the exhaust pipe 6C and the resulting mixture flows into the catalyst 7C. When both the exhaust gas and the heated air flow into the catalyst 7C, the latter is heated by the hot air while, at the same time, HC and CO in the exhaust gas are converted to H.sub.2 O and CO.sub.2 by O.sub.2 in the heated air. In other words, the heat of the hot air and the heat of reaction are effectively used to raise the temperature of catalyst 7C as soon as the engine is started.
If the engine 1C stops running or the state of its running satisfies the present conditions, controll unit 13C closes the control valve 10C while, at the same time, it cause both air pump 8C and heating device 12C to stop operation.
The problem with the conventional exhaust gas purifier having the construction described above is that the durability of the heating device 12C and other devices or components that are located around the heating device 12C is low. This is because when the heating action of the heating device 12C is stopped, air pump 8C is also allowed to stop operating. Hence, even if the application of an electric current to the heating device 12C is cut off, its thermal inertia will warm the heating device 12C and other devices or components that surround it.